The differential amplifier is a very common circuit configuration used to amplify the difference voltage between two input signals. In the ideal case, the output is entirely independent of the individual signal voltages and is dependent only on their difference. Differential amplifiers are widely used in applications where weak signals are to be amplified, particularly weak signals which may be contaminated by common-mode noise. They are universally used in operational amplifiers and are very important in DC amplifier design. Referring to FIG. 1, a schematic circuit diagram is shown for a classic bipolar transistor differential amplifier 10, or "long-tail pair", having a single-ended output.
The differential gain, G, of a differential amplifier and its temperature stability, dG/dT, are typically parameters of great significance in its design and use. The differential mode gain G for the circuit of FIG. 1, taking into account finite current gain (beta) and finite emitter and base resistances for transistors 12 and 14 (which are assumed to be matched), is given by the equation EQU G=R.sub.c /2(r+r.sub.e),
where R.sub.c is the resistance of the collector load resistor 13; r.sub.e is the intrinsic "electronic" emitter resistance, kT/qI, of each of the transistors 14 and 12, where k is Boltzmann's constant, T is the absolute temperature, q is electronic charge, and I is half of the tail current in the lead 18; and r is the total ohmic emitter resistance, r=r.sub.ee' +r.sub.bb' /(beta), r.sub.ee' being the emitter resistance, r.sub.bb' being the base resistance and (beta) being the current gain of each transistor.
Many temperature-dependent factors appear in the foregoing equation. The intrinsic emitter resistance obviously is a function of temperature and perfect stabilization of that quantity requires that the tail current be proportional to absolute temperature (PTAT). The base resistance is very temperature-dependent. The ohmic emitter resistance is also temperature-dependent to a lesser degree. Further, these resistance values are very geometry sensitive and can, with R.sub.c and beta, vary from wafer to wafer (and lot to lot) in the fabrication process.
Accordingly, it is an object of the present invention to provide a differential amplifier with improved temperature stability of gain.
Yet another object of the invention is to provide a differential amplifier in which temperature stabilization and gain uncertainty is minimized from lot to lot in the fabrication process.